Electrical and Computer Engineering ETDs

Publication Date

3-15-1974

Abstract

New processes have been developed for the fabrication of gallium­arsenide-phosphide light-emitting diodes and Schottky barrier field-effect transistors. The fabricated devices were characterized electrically and were tested in a radiation environment. Transient effects on p-n junction diodes due to high energy electron beams were examined and the permanent damage caused by high energy neutrons on both diodes and SBFETs were evaluated.

A unique open-tube diffusion process was developed for the fabrica­tion of the light-emitting diodes. A thick layer of GaAsP-oxide was grown over the epitaxial surface for surface passivation and to act as a diffusion mask against zinc diffusion into the n-type GaAsP. Commercially available Zn-silica film was used as the zinc doping source. The diffusions were performed in an open-tube at 700° C in a non-oxidizing atmosphere of an inert gas. The diffusion time was typically 4 hours. Diffusions performed at higher temperatures caused surface erosion and rendered the epitaxial layer unsuitable for device fabrication. The formation of an intrinsic layer at the epitaxial layer-graded region interface was indicated by the capacitance-voltage measurements. The zero-bias depletion width of the diodes calculated from the experimental data varied from 0.49 microns to 1.07 microns.

The current-voltage characteristics of the diode exhibited the pre­sence of excess leakage current in both the forward and the reverse bias conditions. A large series resistance was also observed in most of the diodes at higher current levels, which was attributed to the presence of intrinsic layers. The reverse breakdown voltage of the diodes was smaller than the theoretically predicted value and the microplasma effect, as well as the sharp edge effects, were identified as possible causes. The diodes exhibited the rectification characteristics in excess of 313° C, which is much higher than operating temperatures of silicon diodes. The leakage current of the diodes was estimated at 5.0 x 10-8 amps, whereas the junction component was approximately 3.0 x 10-10 amps under the same conditions. The activation energy of the leakage current component as determined by the current-voltage characteristics at various temperatures was 0.526 eV.

The electroluminescence from the forward biased p-n junctions was examined for three kinds of gallium-arsenide-phosphide diodes. The phosphorus mole fraction x in GaAs1-xPx for these diodes was 0.3, 0.4 and 0.5, respectively. In addition to peaks in the visible regions, transitions in the infrared and the ultraviolet regions were observed. The efficiency of the light-emitting diodes was in the range of 10-4 percent when the infrared and ultraviolet spectrum were not taken into account.

The transient photocurrent response of the GaAsP diodes, which was generated by an ionizing beam of high energy electrons (0.5 MeV < E < 2.2 MeV) was measured. The electron beam was approximated by a piecewise linear model and the theoretically predicted photocurrent response was compared with experimental results. The effective minority-carrier life­time was determined to be 80 nanoseconds. The generation rate coefficient was calculated to be 1.395 x 1014 carriers/cm3 - Rad(GaAsP). Permanent effects due to the fast neutron irradiation up to fluences of 1 x 1016 neutron cm-2 were examined. The carrier removal rate due to neutron irradiation was 16.3/cm.

The gallium-arsenide-phosphide Schottky barrier RETs were fabricated on a thin epitaxial layer with a semi-insulating gallium arsenide substrate. The epitaxial layer is selectively etched to provide isolation between the devices and to reduce leakage currents in the SFBET. Ohmic drain and source contacts were made by sintering a double layer of indium and gold at 550°C for 15 minutes in an inert gas atmosphere. The contacts were defined by photoresist lifting techniques. The Schottky barrier gate also defined by similar techniques developed with positive working photo­resist.

The formation of an intrinsic layer at the epitaxial layer - substrate interface - was indicated by the three-terminal drain characteristics. The substrate bias effects on device parameters support the above mentioned observation, although the possibility of formation of an intrinsic layer due to sintering step exists. Also a depletion layer could form at the epitaxial layer -semi-insulating substrate interface -due to the sudden bandgap variation or the lattice mismatch. The device parameters like the pinch­off voltage, the channel conductance and the transconductance were measured for the various FETs before and after neutron irradiations. The degradation observed in the parameters was related to pre-irradiation values. The dam­age constants, a and b, defined in Chapter IV, were evaluated to be 4.9 x 10-2 Nn -0·77 and -1.26 x 10-3 Nn -0·64.

Document Type

Thesis

Language

English

Degree Name

Electrical Engineering

Level of Degree

Doctoral

Department Name

Electrical and Computer Engineering

First Committee Member (Chair)

Wayne Willis Grannemann

Second Committee Member

Harold Dean Southward

Third Committee Member

Roy Arthur Colclaser

Fourth Committee Member

William Jackson Byatt

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